The present invention relates generally to switch mode power supplies (SMPS) and, more particularly, to forward mode SMPS that use transformer saturation sensing to control the maximum duty cycle of the SMPS switching elements.
Switch mode power supplies (SMPS) are commonly used to provide DC power to integrated circuits. FIG. 1 illustrates an SMPS 100 in accordance with the prior art. SMPS 100 is coupled to an AC input voltage source 102, and includes a full-wave rectifier 104, bulk capacitor 106, output transformer 108, power supply controller 110, a switch (e.g., a high-power transistor 112, such as a MOSFET), and secondary circuit 114 that includes a filter capacitor 130.
Voltage source 102 provides an input AC voltage between low line and high line values (e.g., between 90 Vac and 264 Vac). Rectifier 104 rectifies the input AC voltage. The output of rectifier 104 is smoothed by bulk capacitor 106, and the resulting input voltage is applied to the primary winding of transformer 108. Bulk capacitor 106 also enables SMPS 100 to maintain the output voltage, VOUT, within regulation for a minimum period of time after the AC power is switched off to SMPS 100. This minimum period of time is commonly referred to as the xe2x80x9chold up time.xe2x80x9d
Transistor 112 is used as a power switch, which is controlled by controller 110. Controller 110 includes a pulse width modulator 116 (PWM), which is driven by a clock 118. A pulse width modulated signal produced by PWM 116 activates and deactivates transistor 112 in response to power supply loading and other control parameters.
Regulation of VOUT is accomplished by varying the on-to-off duty cycle of transistor 112. Essentially, VOUT can be increased or decreased by increasing or decreasing, respectively, the duty cycle of the pulse width modulated signal provided by controller 110.
During steady state operations, when the load applied to the SMPS remains the same, the decrease in the transformer flux when transistor 112 is off must equal the increase in flux when the transistor 112 is on. Otherwise a net change in flux per cycle will occur, and the flux will continue to build up to saturation. Accordingly, if the duty cycle of the pulse width modulated signal is too long, transformer 108 will become saturated. Saturation increases the risk that the power transistor 112 will be damaged by high currents. Therefore, it is desirable to maintain the transformer operation within the transformer""s non-saturation region.
During transient state operations, when the load applied to the SMPS suddenly increases, a longer duty cycle is required to provide the energy necessary to maintain a constant VOUT. This longer duty cycle during transient state operations also must be limited to avoid transformer saturation.
Some prior art systems attempt to avoid transformer saturation by setting a fixed, maximum duty cycle that will avoid saturation during both steady state and transient state operations. Unfortunately, however, these prior art systems have relatively high transistor conduction losses, and require a large bulk capacitor (e.g., capacitor 106) to meet hold up time requirements during power off.
What is needed is an SMPS that efficiently avoids transformer saturation and its associated transistor damage during both steady state and transient state operations. Further needed is an SMPS having a lower transistor conduction loss, and a relatively small bulk capacitor.